1. Field of the Invention
The present invention generally relates to sensors for detecting chemical characteristics of materials and, more particularly, to sensors for use with spectrometric systems.
2. State of the Art
It is often desirable to monitor chemical changes in materials during manufacture. This is especially true in the case of modern composite materials which, if properly manufactured, can have the same strength and stiffness as comparable monolithic materials but with significantly less weight.
In the manufacture of items made from fiberreinforced composite materials, combinations of fiber and polymer resins are employed to form plies, or laminations, which are stacked in predetermined orientations. After stacking, the composite material is cured, usually at high temperatures in an autoclave, to cross-link the polymers in a desired manner. The curing process is usually referred to as "thermosetting". The properties of the composite materials can be selectively altered by changing the fiber and polymer resin matrix components, by changing the fiber orientations, or by changing the manner or extent of cross-linking of the polymer resins. In particular, cross-linking is affected by manufacturing process conditions, primarily temperature and pressure.
Although fiber-reinforced composites can be produced having extraordinary structural characteristics, it is often difficult to develop volume production techniques which reliably produce composite materials having uniform structural properties run-after-run. Frequently, the difficulties in obtaining reproducibility can be traced to inadequate process-control information. Inadequacies in process control information during production can lead to high inspection costs and can result in a substantial portion of a production run being discarded as sub-standard.
In response to the need for process control information during production of fiber-reinforced composite materials, a technique called dielectric cure monitoring has been developed. According to this technique, sensing wires are embedded in a composite material and dieletric permittivity is measured during curing. Dielectric cure monitoring has the advantage that it is a "real-time" process, which is to say that measurements are made simultaneously with the monitored events and, therefore, nearly simultaneous process interventions (e.g., changes in temperature and pressure) can be effected. Unfortunately, dielectric cure monitoring has the drawback that extraneous factors, such as mechanical voids and moisture absorption, often interfere with measurements and cause spurious results. Also, although dielectric cure monitoring is generally thought of as a non-destructive testing technique, embedded dielectric sensing wires can cause micro-cracking of a composite material.
Another technique which has been used to detect conditions of composite materials during thermosetting is Fourier transform infrared (FT-IR) spectroscopic monitoring. This technique is described in a paper entitled "FTIR Characteristics of Advanced Materials" presented by P. Young and A. Chang at SAMPE, Las Vegas, Nev., Apr. 8-10, 1986. In the technique described by Young and Chang, infrared radiation is directed against the surface of a composite material and reflected radiation is collected and measured. By spectrometric analysis of changes in reflected radiation during the cure cycle, Young and Chang were able to determine which wavelengths were absorbed by the monitored material. The spectrometric information was then correlated with cure conditions and, hence, with cross-linking of polymer resins in the composite materials.
A shortcoming of FT-IR cure monitoring as practiced by Young and Chang, supra, is that chemical characteristics were only detected near the surface of composite materials. In practice, surface conditions are not necessarily representative of conditions at substantial depths within items formed of composite materials and, therefore, FT-IR spectroscopic analysis as practiced by Young and Chang does not always fully indicate whether resins are being properly cross-linked (i.e., cured) within the various plies of composite materials.
In view of the preceding, it can be understood that there exists a need for techniques whereby chemical changes, such as during curing, can be directly monitored at substantial depths within an item fabricated from a material such as a fiber-reinforced composite.